The present invention relates to a spin transistor which is a transistor which relies upon two separate populations of charge carriers distinguished by the direction of the spin or magnetic moments of the carriers in the same way that in a conventional semiconductor transistor the carrier families are distinguished by their having different effective electrical charges.
The original spin transistor, or magnetic transistor as it is also known, is described in Science, 260, 320 Apr. 16, 1993, Bipolar Spin Switch by M Johnson. The spin transistor described in this journal report was derived from a metallic, giant magnetoresistance (GMR) magnetic trilayer with contacts to each of three layers. The transistor is biased so as to pump a spin polarized current from one of the magnetic layers into a base layer; the latter suffers a consequent divergence in the chemical potentials of the up and down spin channels and this in turn induces magnetically dependent current flow in a collector (the second magnetic layer). This spin transistor design has a significant problem with its practical implementation in that the transistor offers no power gain. Also, the voltages involved are of the order of nanovolts.
More recent developments in the spin transistor are described in Phys. Rev. Lett., 74, 26, 5260, (1995), D J Monsma et al. The spin transistor described in this journal paper is a metal-semiconductor hybrid in which the rectifying properties of semiconductor junctions are exploited. The transistor consists of two layers of silicon which sandwich a metal GMR multilayer. The transistor is biased to pass current from one silicon layer to the metal multilayer stack and the latter's magnetic configuration then governs what proportion of the current eventually penetrates to the second silicon layer. In this respect, the multilayer stack operates as a normal magnetic multilayer which has a large resistance in a low magnetic field and a small resistance in a high magnetic field. The current gain of the transistor is magnetically controllable by a factor of around 2, with the GMR of the multilayer on its own being only about 3%. Even with this design, however, while the collector/base current gain .beta. varies by a large factor, its actual value is very small and, in fact, is less than unity, whereas for a commercial silicon transistor .beta. is generally 200 or more.